1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission.
2. Description of Related Art
Many kinds of automatic transmissions comprise a torque converter and a multiple stage shift gear mechanism coupled to the torque converter. The multiple stage gear mechanism performs a shift by shifting operational states of friction coupling elements such as clutches and brakes mounted thereto. Shifting the operational states of the friction coupling elements is performed on the basis of a predetermined shift characteristic which is set generally using a vehicle speed and an engine load as parameters.
When the engine is cold, a driving state of the engine may not be rendered stable, so that Japanese Patent Examined Publication No. 32,941/1980 proposes that the shift characteristic be transferred to the high speed side during a so-called cold time period when the temperature of an operating oil is below a given value. Hence, it is proposed that a driving range be widened, in which the driving is performed at a low speed stage over the cold time period.
It is significant from the viewpoint of a prevention of shift shock, too, how to set the line pressure of the operating oil for the friction coupling elements. Therefore, generally speaking, for instance, the line pressure is subjected to duty control, while the duty ratio for the duty control is corrected in accordance with the oil pressure.
During the cold time when the oil is below the freezing point, the viscosity of the operating oil may vary to a greater extent, as compared with an ambient atmosphere where the operating is not cold, so that it may be impossible to control the line pressure to a desired magnitude with the aid of the duty control even if the duty ratio would be corrected. When the line pressure is controlled by the process other than the line pressure control, an increase in the viscosity of the operating oil may change a timing for shifting, thereby worsening the shock of a shift. Therefore, it is important to raise the temperature of the operating oil and depart from a cold state as soon as possible in order to prevent the shock of the shift.